1. Field of the Invention
The present invention relates to an optical scanner having a vibration mirror used for a laser raster writing optical system and a color image forming apparatus.
2. Description of the Related Art
Up until now, in order to allow a color image forming machine to perform high-speed printing and realize high image quality, it is required that a polygon scanner rotate at a high speed of 25000 rpm or more and with high accuracy. On the other hand, the high image quality can be obtained with the reduction of the diameter of a laser beam. Therefore, the radius of the inscribed circle and the length in the main scanning direction of a polygon mirror used for the polygon scanner become relatively large, which results in an increase in the load of the polygon scanner.
When the load of the polygon scanner increases, its consumption power increases and heat generated by the polygon scanner adversely affects optical elements such as a scanning lens. Specifically, the temperature of the scanning lens closest to the polygon scanner increases. The heat generated by the polygon scanner is transmitted or radiated to an optical housing, which increases the temperature of the scanning lens. However, the increase in the temperature of the scanning lens is not uniform. That is, the temperature distribution of the scanning lens is concentrated particularly in its main scanning direction as a longitudinal direction based on a distance from the heat generation source (polygon scanner) or a difference between the thermal expansion coefficients of materials and the influences of airflow.
If the scanning lens has the temperature distribution that is mostly concentrated in the main scanning direction, its shape accuracy and refractive index are changed, which in turn particularly fluctuates the spot position of a laser beam and degrades the quality of an image. This problem is pronounced in plastic having a large thermal expansion coefficient.
The color image forming machine performs scanning with laser beams corresponding to colors (yellow, magenta, cyan, and black). Therefore, besides the above problem, a temperature deviation between optical scanners corresponding to the respective colors is generated. The temperature deviation causes the shift of a relative positional relationship between the spots of beams corresponding to the respective colors, which results in the color shift of an image.
In addition, with the increase in the temperature of the polygon mirror having a large load, small movements of components (particularly the polygon mirror having a high mass ratio) constituting a rotating body are induced. As a result, the rotating body gets out of balance and generates vibrations. If the thermal expansion coefficients of the components (the polygon mirror, a flange to which a rotor magnet is fixed, and a shaft) constituting the rotating body are different, or if the tolerances of the components, a method for fixing the components, etc., are not strictly managed and inspected even where the thermal expansion coefficients of the components are the same, the rotating body generates small movements (gets out of balance) when rotating at high temperature and high speed, and thus increases vibrations. The vibrations are transmitted and amplified to the optical element (such as a return mirror) of the optical scanner. As a result, banding is generated to cause the degradation of an image and noises.
In view of this problem, Patent Document 1 has proposed an optical scanner in which the polygon mirror is replaced by a vibration mirror (resonant mirror). With this optical scanner, a beam is detected by a synchronization detection sensor and a terminal detection sensor that is arranged near a part having a scanning angle θ0, and a time difference between a synchronization detection signal and a terminal detection signal is calculated by an amplitude calculation unit. Thus, the deflection angle (amplitude θ0) of the resonant mirror is detected. Here, assuming that the scanning angle of the optical beam detected by the sensors is θd, a scanning time from the center of an image is t, and the driving frequency of the resonant mirror is fd, the formula θd/θ0=sin 2π·fd·t (wherein t=T/2) is provided. The deflection angle of the resonant mirror is corrected in such a manner that the gain of a voltage pulse applied is varied until this time difference T reaches a predetermined reference value T0. Accordingly, even if there is a fluctuation in the number of resonant vibrations inherent in the vibration mirror, joints are unnoticeable. As a result, it becomes possible to perform high-definition image formation without causing color shift and discoloration.
However, the above configuration requires plural (m pieces) resonant mirrors for each color and that requires m×4 pieces of the resonant mirrors so as to be compatible with colors (tandem arrangement). Because the joints per se still exist even if the deflection angles of the resonant mirrors are corrected, the degradation of an image cannot be eliminated in principle. Moreover, the phases of the resonant mirrors between the colors are controlled with the arrangement of plural of the resonant mirrors for each color in the tandem arrangement. However, the configuration cannot correspond to a driving signal that drives the resonant mirrors according to a temperature variation and a pressure variation in time and environment that cause variations in the phase of the resonant mirror. As a result, the degradation of an image (color shift and color shading) is caused.    Patent Document 1: JP-A-2004-279947